Accelerator Transmutation of Waste

Jamie Ray
March 11, 2011

Introduction to Nuclear Waste

Continued development of nuclear fission as a source
of energy is blocked by three main fears: potential reactor meltdown,
proliferation of weaponizable fuel, and unsolved buildup of nuclear
waste. As it turns out, the least progress has been made on addressing
the waste issue! The concern with nuclear waste is its radioactivity -
radioactive elements are unstable and can decay into other elements,
releasing energy in the process. In some cases this energy can be
threatening to humans or other life close enough to be exposed to the
radiation. Waste is categorized according to two metrics - the danger it
poses, and the lifetime over which it remains dangerous. Some
radioactive waste is a byproduct of medical and research efforts, but
the majority is produced through nuclear fission in reactors. [1]

The fuel in a nuclear reactor consists of heavy
elements, most often U-235 and U-238. This fuel is constantly bombarded
by neutrons during the reactor's operation, which can generate new
elements through two mechanisms. The first is nuclear fission - a
'fissionable' element may split after it is hit by a neutron. This
process, which occurs for the U-235, releases energy and some more
neutrons as the nucleus divides into several (often two) smaller nuclei.
The resulting elements are called 'fission products' (FP). However,
neutrons colliding with U-238 will not split the nucleus, and are
instead absorbed to create a new isotope of the element with one more
neutron. Elements created through absorption rather than fission
therefore have a much higher nuclear mass, and are known as actinides,
or 'transuranics' (TRU) if heavier than uranium. Over the course of a
reactor's lifetime, many fissions and absorptions occur in various
combinations so that the fuel contains an incredible diversity of
elements when removed from the reactor core. Many elements created
through fission or absorption are very unstable and will quickly decay
into more benign species, but a few pose a serious threat over longer
timescales, even though produced in small quantities. These products
make a solution to long-lived radioactive waste integral to any nuclear
energy proposal.

Waste 'Solutions'

Several solutions have been proposed and enacted for
dealing with nuclear waste. In a 'closed' fuel cycle, the spent fuel
from a reactor is reprocessed to extract the remaining usable elements.
The unusable radioactive waste is left behind in a more concentrated
(usually liquid) form. [1] In contrast, the 'open' fuel cycle does not
reprocess spent fuel so while it contains much the same spectrum of
radioactive elements, they constitute only a small amount of the total
mass, and the fuel is left solid. In either form, the waste produced
contains the same radioactive elements that have to be dealt with.
Initially, the waste is so highly radioactive that it can kill in
minutes and some of the radioactivity heats it to high temperatures. [2]
It is usually put in 'interim' storage, often in sealed containers that
are kept underwater to cool it down. While this isn't ideal and the
storage must be kept under surveillance, simply waiting for a few years
will decrease the waste's potency. By then, the most radioactive
'short-lived' elements will have mostly decayed, and the truly
problematic elements remain. They include both long-lived fission
products and actinides, seen in Table 1. [1,3] In waste that hasn't been
reprocessed, there is also a considerable amount of plutonium (a TRU) in
different isotopes, all radioactive.

There are two options to avoid the threat of the
longer-lived radioactive elements. One is to wait until they decay into
stability, the same strategy as is used during the interim storage.
However, current interim storage isn't suitable for the much longer
timescales necessary for waste to become safe. As a result, the concept
of 'geologic' storage has been proposed, in which large amounts of waste
are stockpiled and sealed away for millions of years, with a thick layer
of rock (hence the name) separating them from us. [1] With thousands of
tons of nuclear waste, the United States has investigated storage in
Yucca Mountain since 1982, but little progress has been made and in the
meantime the waste remains in interim situations that weren't meant to
be used for decades. [2] Geologic storage promises a very long wait,
however, and the prospect of continually searching for and filling new
repositories as more waste is produced. The other option is to
accelerate the change of dangerous waste into more benign elements by
transmuting it with more neutrons. A fission reactor is unsuitable for
this purpose because the waste acts as a 'poison' preventing chain
reactions from occuring. Other sources of neutrons are possible,
however, including those from nuclear fusion and created by particle
accelerators. The last is known as 'accelerator transmutation of waste'
(ATW) and is being investigated as an alternative to the geologic
storage solution.

ATW Concept

The basic concept of ATW is simply to 'finish the
job' of burning or transmuting the radioactive waste through intense
neutron flux, thereby drastically reducing its radioactivity. Again,
bombardment with large numbers of neutrons is considered as a way to
either fission the heavier elements into less dangerous species or
convert the lighter ones through absorption. A particle accelerator can
be used as a source of high-energy protons, which can then be used to
create neutrons through a process known as 'spallation.' This occurs
when a proton collides with a heavy nucleus in the spallation target,
after which the nucleus ejects a large number of neutrons proportional
to the energy of the original proton. [4] However, advances both in the
technology needed to accelerate the protons and the understanding of
spallation physics have prompted investigation of accelerator-created
neutrons for waste treatment. [4] While doing so may create further
radioactive elements both in the spallation target and in the bulk of
the waste, by eliminating the long-lived ones from Table 1 above ATW can
theoretically change the nature and the timescale of the problem
considerably.

The question is, how many and what kind of neutrons
are necessary for effective transmutation? The answer depends on the
cross sections of the waste elements, where cross section relates to the
probability to absorb or fission when hit by a neutron of a given
energy. For a low cross section, one must generate more neutrons and use
more energy to transmute the offending element. The TRU elements
remaining in the waste are generally fairly easy to fission, but the
threatening FP's usually have much lower cross sections for the desired
neutron absorption that can transmute them into a more benign state. [4]
These elements present a greater concern than the TRUs because they can
easily contaminate water and so leaks of radioactive waste can spread
much more quickly if they include long-lived FP's. To transmute them
effectively, ATW systems propose using a large flux - about
1016 cm-2 sec-1
- of relatively low-energy 'thermal' neutrons that
can also fission the TRUs. [5,6] To acheive the desired flux, ATW
proposals usually include an accelerator that can produce 1 GeV protons.
One problem is that, with a goal of waste reduction in mind, the most
effective system will be one that bombards the most concentrated waste.
A target with a high proportion of non-waste elements (like the leftover
uranium in 'open' cycle spent fuel) will have a lower chance of hitting
an FP or TRU with a given neutron, thus lowering the 'effective' neutron
flux. Of course, one could process the spent fuel to separate the
uranium and plutonium, but the US and some other countries have chosen
the open cycle specifically to avoid reprocessing, which they claim
increases proliferation risk. Instead, proponents of ATW have largely
turned to an attractive alternative that takes advantage of the large
amount of usable fuel mixed in with the waste.

ADS and EA proposals

The proposal that now dominates work in ATW is to
combine it with the concept of the Accelerator Driven System (ADS). An
ADS is a kind of nuclear reactor that is 'driven' by neutrons from a
particle accelerator. The rate of reactions in a nuclear plant is
determined by the flux of neutrons within the fuel, so by using an
external source of neutrons the ADS is proposed as a way to control the
rate more effectively. The accelerator doesn't provide all of the
neutrons used for fission, only a fraction that can then set off chain
reactions and thereby create more neutrons for reactor operation.
Instead of the single goal of transmuting the waste, which can be
prohibitively expensive in the unprocessed form, we now have added the
goal of generating energy through fission of usable fuel. Such a
combination has been suggested both by the Los Alamos National Lab and
the former director of CERN, Carlo Rubbia, who calls it an 'Energy
Amplifier' or EA. [7] While a traditional ADS might use fuel similar to
a nuclear reactor, the EA relies more on 'fertile' elements that can be
transmuted or 'bred' into fissionable fuel, like the high proportion of
U238 which can be bred into Pu-239.

A large number of energy-producing ATW designs have
been proposed varying by type of fuel, processing required, accelerator
and spallation technology, and neutron spectrum. Transmuting the
long-lived TRUs seems to be acheivable in such a scenario, but dealing
with the fission products falls by the wayside. [6] This is partly
because the fission products are what cause regular reactors to stop
working in the first place, acting as a 'poison' that consumes too many
neutrons and slows the reaction down. To really deal with the waste
economically, all scenarios assume some kind of processing - otherwise,
far too many neutrons would be required because so many would be wasted
in useless capture processes. The scheme is to use reprocessing
technology that is 'proliferation resistant' in that it avoids
generating a stream of pure weapons-grade plutonium, and run an ADS
using TRUs and only the most dangerous FPs from Table 1. The TRUs are
used as fissionable fuel, thereby reducing their danger as waste and
extracting the extra energy that makes them dangerous in the first
place. Fission products like iodine and technetium are separated during
processing and included either in the fuel or near it to absorb some of
the neutrons and be transmuted. [3] Originally, producing energy was
proposed as a way to offset cost and work with open cycle spent fuel,
but in recent ATW proposals it has taken over as the priority, so that
actual treatment of the waste isn't optimized. Energy generation may
help with costs, but it makes the solution of the waste problem less
complete and requires reprocessing and new technologies that haven't yet
been tested.

Concerns and Future Progress

Much testing needs to be done to demonstrate the
feasibility of ATW. It relies on advanced accelerator and spallation
technology, and energy production would require new processing methods
that also haven't been used yet. [3] For a facility proposed to
transmute U.S. Waste, the theoretical prediction is that "after 60 years
of operation, nearly all of the transuranics will have been fissioned,
most of the technetium will have been converted to stable isotopes of
ruthenium, and most of the iodine will have been transmuted to stable
isotopes of xenon." [3] GeV-scale proton accelerators have been demonstrated, but no
existing versions have the reliability, energy efficiency, and high
throughput (or current) that is assumed - often a linear accelerator -
in ATW models. As a result, the type of long-term spallation targets
needed, which often consist of a lead-bismuth (LBE) alloy, also haven't
been tested to the extent that they would be used. The processing
technology, combinations of pyroprocessing and fluorination have been
considered, have also not been implemented on the scale necessary.
[6]

Although these issues of technological implementation
are important, the biggest questions are whether energy can actually be
generated, and how effectively the FPs will be transmuted. Among a wide
range of accelerator parameters, a popular choice seems to be a 1.6 GeV
proton linear accelerator with a 250 mA current - the best existing
linear accelerators achieve about 1 mA of current. [8] This means that
the beam energy would be 400 MW with a total of 1.6 ×
1018 protons per second hitting the spallation target. At
existing levels of accelerator technology, the most economical equipment
might be 30-50% efficient: a Los Alamos paper on this design suggested a
figure of 900 MW of electricity used to run the accelerator. [8] Another
proposal, based on a U.S. 'Roadmap' for the ATW program, claims to use
the accelerator much more sparsely, with a 90 mA beam of protons at 1GeV
consuming around 300 Mwe. [3] It purports to generate around 1500 Mwe
with such a beam by fissioning TRUs using several ideal assumptions:
high thermodynamic efficiency (37%), negligible processing costs, high
neutron multiplication (~33), and the accelerator supplying a constant
flux of neutrons. [3] None of these assumptions are guaranteed or
demonstrated in current systems; moreover, the report claims to use .65
neutrons per fission event to transmute the FPs iodine and technetium!
While it's easy to say that the long-lived fission products will be
burned, the fact is that they have much lower cross sections than the
TRUs so that it is ridiculous to assume that their absorption rate will
be comparable to the fission rate, unless a much larger volume of FPs is
placed in the reactor. [9]

Essentially, current ATW proposals sacrifice
effective transmutation for energy, and require several unproven
technologies combined with concepts used in Generation IV breeder
reactors like advanced processing and molten coolants. The idea of using
accelerator neutrons to manipulate the reactor's neutron economy is a
nice one, and indeed energy might plausibly be produced through burning
the TRUs, but it is not a complete waste solution (especially in the
case of long lived fission products) and requires significant
engineering advances in accelerator, spallation, cooling, and
reprocessing technology. Even if energy can be produced, there is little
evidence that a combined system would be more effective than simply
burning the TRUs in an ADS and then using the energy for a separate,
dedicated ATW system. Even this scenario would require detailed
processing of the wastes to high purities that hasn't been
demonstrated.